This is based off of #11622 and adds a deprecation warning when a BIP70 URL is used.
Rational:

BIP70 increases attack surface in multiple ways and is difficult for third party wallets to implement in a secure manner
Very few merchants use the standard BIP70 variant supported by Bitcoin Core
The one major payment processor that doesn't support BIP21 and currently uses a customized non-standard version of BIP70 has indicated that "Unfortunately the original BIP70 is not useful for us."